In the above-mentioned co-pending application, a coupling ring is disclosed for receiving and joining circular air ducts. The coupling ring comprises a sheet metal strip having a pair of lateral flanges joined by a channel-shaped rib which overhangs the flanges and defines therebetween oppositely opening lateral grooves. The strip is formed in a roll forming machine from flat metal stock. A length of the strip is bent into a coil with the rib on the outside and the ends of the strip are brought together and secured as by welding to a butt block. When it was attempted to coil the strip with the rib on the outside of the coil, it was discovered that the oppositely opening lateral grooves were substantially distorted preventing proper entry and seating of the ends of the ducts and resulted in an unsatisfactory joint between the ducts. Distortion of the oppositely opening lateral grooves was also a serious drawback because in use the coupling ring has sealant material disposed in the grooves and the ends of the ducts are simply to be buried in sealant and then secured to the flanges with fasteners. With distorted grooves the sealant does not properly cover the ends of the ducts and air leaks occur.
Various attempts were made to coil the strip, and prior art machines were considered for possible use in this connection. A search of the prior art uncovered the following patents, none of which discloses a machine suitable for coiling the strip:
U.S. Pat. No. 2,257,760 PA1 U.S. Pat. No. 2,775,284 PA1 U.S. Pat. No. 3,319,445 PA1 U.S. Pat. No. 3,472,056 PA1 U.S. Pat. No. 4,080,815 PA1 U.S. Pat. No. 4,562,630
In analyzing the distortion occurring in the oppositely opening lateral grooves during attempts to coil the strip, it was noted that the rib was subjected to tensile stresses while the lateral flanges were under compression. These oppositely directed stresses within the strip are focused close to the oppositely opening grooves during the coiling and resulted in distortion of the grooves.